Check out Bob Monaghan's Bronica website for info on all things classic Bronica.
I have a pretty extensive Bronica system built around the S2A, and I think it offers a great value for the price. One issue with the EC is that the split mirror can become misaligned and becomes impossible to repair if it does, so you want to be sure it's okay before purchasing. The EC and EC-TL offer mirror lockup, which does not exist on the S-series Bronicas, and some users feel that the EC is a step forward in design, while the S2A merely fixed all the problems with the S2 and earlier cameras by using stronger, heavier parts. That said, I like the ruggedness of the S2A. Lenses are interchangeable between the S-series and EC-series, but finders and backs are not.